Disclosed herein are novel, stable aqueous particle dispersions. The polyurethane dispersants that produce the stable aqueous particle dispersions, especially pigment dispersions, the process of making the pigment dispersions, and the use thereof in ink jet inks are also disclosed herein.
Polyurethanes are materials with a substantial range of physical and chemical properties, and are widely used in a variety of applications such as coatings, adhesives, fibers, foams and elastomers. For many of these applications the polyurethanes are used as organic solvent-based solutions. However, recently environmental concerns have caused solvent-based polyurethanes to be replaced by aqueous dispersions of polyurethanes in many applications.
Polyurethane polymers are, for the purposes of the present invention, polymers wherein the polymer backbone contains urethane linkage derived from the reaction of an isocyanate group (from, e.g., a di- or higher-functional monomeric, oligomeric and/or polymeric polyisocyanate) with a hydroxyl group (from, e.g., a di- or higher-functional monomeric, oligomeric and/or polymeric polyol). Such polymers may, in addition to the urethane linkage, also contain other isocyanate-derived linkages such as urea, as well as other types of linkages present in the polyisocyanate components and/or polyol components (such as, for example, ester and ether linkage).
Polyurethane polymers can be manufactured by a variety of well-known methods, but are often prepared by first making an isocyanate-terminated “prepolymer” from polyols, polyisocyanates and other optional compounds, then chain-extending and/or chain-terminating this prepolymer to obtain a polymer possessing an appropriate molecular weight and other properties for a desired end use. Tri- and higher-functional starting components can be utilized to impart some level of branching and/or crosslinking to the polymer structure (as opposed to simple chain extension).
Polyurethanes have been prepared from diols as disclosed in Statutory Invention Registration US H2113 H but with the limitation that the polyurethane has a hydroxyl number greater than 10 and thus the polyurethanes described are not urea terminated. Polyurethane have been prepared from polyether diols as disclosed in EP1167466, US2004/0092622 and US2003/0184629 but these polyurethanes are chain extended with di or triamines, which will result in a polyurethane which has been bridged by the di or triamine chain extension. US2004/0229976 describes the use of polyurethane resins as freely added materials in pigment-dispersed aqueous recording liquid which have at most 2.0 wt % of polyurethane urea in the polyurethane.
Polyurethanes with both monofunctional end-capping and chain extension of the polyurethane have been described in WO2006/027544.
Polyurethanes have also been prepared using polytrimethylene ether glycol (PO3G) based homo and copolymers, as disclosed in U.S. Pat. No. 6,852,823, U.S. Pat. No. 6,946,539, US2005/0176921A1, U.S. application Ser. No. 11/294,850 (filed Dec. 6, 2005), and Conjeevaram et al. (J Polym Sci, 23, 429, (1985)), the disclosures of which are incorporated by reference herein for all purposes as if fully set forth. The most common source of PO3G and its precursors are from biosynthetic pathways that are described in the aforementioned patents and applications. Polyurethanes derived at least in part from biosynthetic pathways are important nowadays, as they reduce our reliance on the petrochemical industry.
Aqueous dispersions of self-dispersing, ionic polyurethanes have also been proposed, for example, in U.S. Pat. No. 3,412,054 and U.S. Pat. No. 3,479,310, the disclosures of which are incorporated by reference herein for all purposes as if fully set forth. In these disclosures, ionic and/or non-ionic or potentially ionic diols are incorporated into the polyurethane polymer and, following neutralization, these polyurethane ionomers can be stably dispersed in water.
Typically, polyurethane dispersions have been made using a wide range of polymeric and low molecular weight diols, diisocyanates and hydrophilic species. The dispersion process may involve synthesis and inversion from volatile solvent such as acetone, followed by distillation to remove organic solvent components. Polyurethanes may also be synthesized in the melt phase with or without inert, non-volatile solvents such as NMP (N-methylpyrrolidone). In this case, the solvent remains in the polyurethane dispersion. Added emulsifiers/surfactants may also be beneficial to dispersion stability.
Recently, polyurethane dispersions have been extended to acrylic/polyurethane hybrids and alloys, such as disclosed in U.S. Pat. No. 5,173,526, U.S. Pat. No. 4,644,030, U.S. Pat. No. 5,488,383 and U.S. Pat. No. 5,569,705, the disclosures of which are incorporated by reference herein for all purposes as if fully set forth. This process typically involves synthesis of polyurethanes in the presence of vinylic monomers (acrylates and/or styrene) as the solvent. Following inversion to form a polyurethane dispersion, the acrylic or styrenic monomers are polymerized by addition of free radical initiator(s). Variations on this process are known in the art. Acrylic/urethane hybrid dispersions offer potential advantages to coatings and other end products, including enhanced hardness, adhesion and nearly Newtonian rheology along with lower cost, low VOC and improved manufacturing.
Polyurethane dispersions that are used as pigment dispersants have been described in U.S. Pat. No. 6,133,890. These polyurethanes are prepared with an excess of isocyanate reactive group and are limited to the presence of polyalkylene oxide components. Aqueous polyurethane dispersants have found limited use as dispersants for pigments and the like.
Therefore, there is still a need for a new class of polyurethane dispersants that can stably disperse particles, especially pigment particles, in aqueous medium, and are especially suited for use in aqueous inkjet inks. It would also be advantageous for such class of polyurethane dispersants to be capable of being derived from environmentally favorable (“green carbon”) materials and of being formulated into dispersion in a convenient environmentally friendly manner.
None of the above publications disclose polyurethane dispersants derived from water dispersible urea terminated polyurethanes based on certain diols and polyether polyols which have at least 3 but less than 30 (substituted) methylene groups in the diol. It has been discovered herein that these novel polyurethanes dispersants can be used as a dispersants for pigments, especially pigments for inkjet inks, and have the forgoing unique combination of attributes.